1. Field of the Invention
The present invention relates to a high-strength steel wire and a method for production thereof, said steel wire being one which is ready for shipment without heat treatment (such as blueing) after cold working and which finds use for steel cords and wire ropes.
2. Description of the Related Art
Automotive steel tires are reinforced with steel cords or bead wires, which are composed of very thin steel wires twisted together, each being about 0.15 to 0.4 mm in diameter and having high strength in excess of 310 kgf/cm2.
Said steel wire is produced from a hot-rolled wire rod of high-carbon steel (eutectoid steel or hyper-eutectoid steel) by drawing (for reduction in diameter), patenting, acid pickling, brass plating (for metal lubrication), and final wet cold drawing. The resulting steel wire is as thin as about 0.2 mm in diameter. The patenting step is carried out at about 500-550xc2x0 C. so as to transform the austenite structure into the uniform, fine pearlite structure, thereby imparting toughness to the steel wire.
Recent automotive tires are required to have improved durability, and steel wires for tire cords are required to have higher strength than before. Steel wires can be improved in strength readily by increasing the carbon content. However, high strength should be accompanied by sufficient ductility. Any attempt to improve strength without respect to ductility ends up with a problem with longitudinal crackingxe2x80x94fracture that occurs in the lengthwise direction upon twisting.
Several ideas have been proposed as follows to prevent longitudinal cracking.
Japanese Patent Publication No. 99746/1994 discloses a steel incorporated with Cr and Co which make the pearlite lamellar structure fine.
Japanese Patent Laid-open No. 99312/1997 discloses a method of drawing a steel wire continuously through a die in such a way that the reduction of area is controlled in response to the amount of strain due to drawing.
Japanese Patent Laid-open No. 121199/1998 discloses a steel wire composed mainly of fine pearlite, with its lamellar cementite rendered amorphous.
Japanese Patent Laid-open No. 199980/1999 discloses a steel wire having the pearlite structure such that ferrite contains no more than 1.5 atom % of carbon dissolved therein.
Japanese Patent Laid-open No. 269607/1999 discloses a steel wire in which the amount of cementite is controlled in response to the amount of carbon and the average particle diameter of cementite is 2-10 nm.
The above-mentioned prior art technology has achieved to some extent the object of improving strength. There still is a need for further improvement in strength. Unfortunately, a high-carbon steel wire suffers strain aging when it is allowed to stand at room temperature after drawing, and this strain aging increases strength further. [See xe2x80x9cZairyou to Purosesuxe2x80x9d (Materials and Processes) CAMP-ISIJ vol. 12 (1999), p. 461.] Increase in strength due to strain aging makes a high-carbon steel wire more vulnerable to longitudinal cracking. This has stimulated the development of a high-strength high-carbon steel wire which has ductility enough to retain good resistance to longitudinal cracking even though strength increases due to strain aging.
The present invention was completed in view of the foregoing problem. It is an object of the present invention to provide a high-strength steel wire and a method for production thereof, said steel wire having high strength as well as sufficient ductility and excelling in resistance to strain aging embrittlement and longitudinal cracking.